A disk drive is a data storage device that stores digital data in tracks on the surface of a data storage disk. Data is read from or written to a track of the disk using a transducer that is held close to the track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo scheme is generally implemented that uses feedback data read from the disk surface to align the transducer with the desired track. The servo data is written to the disk using a servo track writer (STW).
In an ideal disk drive system, the tracks of the data storage disk are non-perturbed circles situated about the center of the disk. As such, each of these ideal tracks includes a track centerline that is located at a known constant radius from the disk center. In an actual system, however, it is difficult to write non-perturbed circular tracks to the data storage disk. That is, problems (such as inaccuracies in the STW and disk clamp slippage) can result in tracks that are written differently from the ideal non-perturbed circular track shape. Positioning errors created by the perturbed nature of these tracks are known as written-in repetitive run-out (STW_RRO). The perturbed shape of these tracks complicate the traducer positioning function during read and write operations because the servo system needs to continuously reposition the transducer during track following to keep up with the constantly changing radius of the track centerline with respect to the center of the spinning disk.
In certain conventional systems, as will be understood by those skilled in the art, the STW is used to directly measure the STW_RRO for each track of a disk so that compensation values may be generated and used to position the transducer along an ideal track centerline. In such systems, the STW must measure the STW_RRO of each track of a disk one track at a time. Because (1) a typical disk drive has four or more disks, (2) a typical disk contains over 10,000 tracks per inch (TPI) and (3) typical disk rotation speeds are around 5400 revolutions per minute (RPM), the STW could be tied-up for several hours in measuring the STW_RRO for one disk drive. The values of the STW_RRO for each track (or section of track) are then stored on the disk for use during transducer positioning. For an example of a disk drive system that is similar to the above described system, reference is made to U.S. Pat. No. 4,412,165 to Case et al. entitled "Sampled Servo Position Control System".
As is well-known in the art, STW's are very expensive and, therefore, only a limited number of STW's are available at a disk drive manufacturing facility. Accordingly, by tying-up the STW's for extended periods of time in measuring the STW_RRO for each disk drive, the manufacturing throughput/efficiency will be dramatically decreased.
Therefore, it would be advantageous if a system were provided for compensating for the STW_RRO with ring the use of a STW to determine the STW_RRO so that such system could be implemented in a high volume production environment.